1. Field of the Invention
This invention relates to a display element which may be used alone as an indicator but will be more commonly used as a pixel in an array of such elements to form a changeable display sign. The invention will be found particularly useful for signs and displays which must be viewed over wide angles such as bus destination signs.
2. Summary of the Invention
The display element is of the type using a rotor disk defining a median plane, having opposed sides which respectively contrast and conform with their background and which is rotatable about a rotation axis approximately parallel to said median plane to display the bright or dark sides of the disk in a viewing direction which is considered the axis of a viewing cone (not necessarily a surface of revolution) which surrounds the viewing direction. The stator which forms the background to each disk is coloured darkly to contrast with the disk bright side; and conform to the disk dark side. The bright and dark sides are displayed in the ON and OFF respective positions.
A light emitting diode (LED) corresponding to each disk is positioned to form part of the disk's pixel when the bright side is displayed (called the ON position) the LED being positioned and directed to illuminate the bright side of the disk when oriented to ON positions The LED is permanently on and so must be masked to the viewer in the OFF position of the disk.
`Forward` and `rearward` are respectively, the directions from the display element toward the viewer, and the opposite direction.
An `array` is the entire bus sign or other sign, composed of `sub arrays` which are each made up of a column of individual display elements, or of individual elements.
Reference to an `LED` herein is intended to include a cluster of such LED's.
The `viewing direction` is the general centre of the locations (projected on a plane perpendicular to the viewing direction) from which the display element, or any array thereof, is intended to be viewed.
The `viewing cone` surrounds the viewing direction and includes the projections on such plane of the positions from which the display elements or an array thereof is intended to be viewed.
In a preferred aspect of the invention herein the angle through which the illuminated disks (in plan view) may be viewed is about 150.degree. approximately symmetrically disposed about the longitudinal axis of a bus on which the array is disposed. However, the direct light from the LEDs may be viewed over an arc of about 75.degree. from the longitudinal axis on the side of the bus toward which the light from the LED is directed. This is useful since a bus sign is often viewed from the sidewalk. Thus, for buses for use in most countries other than England and Australian the 75.degree. sector will be on the right of the bus center line, and in England and Australia on the left.
It is known to use such a disk augmented by the end of an optic fibre. See for example patents:
U.S. Pat. No. 4,974,353 dated Dec. 4, 1990, Norfolk PA1 U.S. Pat. No. 5,022,171 dated Jun. 11, 1991, Norfolk, et al PA1 U.S. Pat. No. 5,055,832 dated Jun. 8, 1991, Browne
However, optic fibres while suitable for relatively narrow angle viewing are not so suitable for viewing over wide angles, as are LED's. Moreover, LED's are cheaper to a sufficiently marked degree, that a display application with LED's may be practical where a similar application with fibres would be impractical.
Other patents have used LED's with a rotating disk. See for example, U.S. Pat. No. 5,050,325 dated Sep. 24, 1991. However this patent does not provide for masking of the LED by the disk per se,nor for wide angle viewing. Hence the LED had to be switched off in OFF state. The design of the present display element assumes that the LED will be continuously on while the disk switches between ON and OFF positions, thus avoiding the cost of individual switching circuitry for each LED. Such switching requires design complexity and expenses
Accordingly, it is an object of this invention to provide a display element, for use alone or in an array of such elements, wherein the appearance of a rotatable flip disk is augmented in ON orientation by an LED, which LED must be masked by the disk, to the viewer, in OFF orientation and wherein the rays from the LED are directed to illuminate only the bright side of the disk with which it is associated and to be viewed directly
It is an object of this invention to provide a display element or an array thereof allowing, in ON position, viewing over a wide angular range and in OFF position masking the rays over a wide angular range, and preferably where the rays reflected from a disk illuminated by the LED are visible through a sector in plan view which is symmetric with regard to the forward direction in plan view.
By `disk viewing direction` herein I mean the direction approximately perpendicular to the average attitude of the illuminated disk in ON and OFF position and perpendicular to its rotary axis. This will not be parallel to the disk in ON and OFF position since the ON and OFF disk orientations are slightly offset from each other to reduce rotation below 180.degree. where a magnetic drive is used, for reasons associated with the drive. Where an array of display elements is used, the viewing direction is taken as perpendicular to the plane approximating the locus of the array.
By `plan view` I mean the view in the direction perpendicular to the viewing direction and parallel to the median plane of an array. Plan view in relation to a bus will therefore be approximately a vertical view but in other applications it may have any real orientation.
It will be realized from what has already been said that for a bus or other vehicle destination sign, the plan view will be in, approximately, a horizontal plane and that the viewing locations for viewing directly LED light will be all on one side of light viewing direction for the illuminated disks.
The invention in one aspect provides a display element suitable for use as a pixel in an array, having a disk movable between an ON position (displaying a bright side to a viewer) and an OFF position (displaying a dark side to the viewer) with an LED positioned to illuminate the bright side of the disk in ON position, and where the element or an array thereof may be located in immediate proximity to a bus (or other vehicle) windshield to be viewed by pedestrians there-through. The proximity of the display element to the windshield is limited only by the spacing required to allow movement of the disk between ON and OFF positions and static charge interaction between disk and windshield.
The closer the array is to the windshield, the wider may be the viewing angle.
It is an object of this invention to provide a display element wherein the light source used is markedly maintenance free and inexpensive in contrast to alternate designs.
It is an object of this invention to provide a display element for forming a pixel which is suitable for relatively large multiple pixel arrays (for example, of 20 by 40 pixels) to provide good definition in the sense of providing a small pixel relative to the size of the array and a pixel having a relatively large effective area.
By `effective area` I mean the percentage of the sign area which is occupied by the bright areas of the disks when all are ON. This is a measure of the sign's efficiency even though it may not be strictly accurate in view of the effects of the light effects of the LED's and the preferred angle of the disk bright panels to the viewing direction. The need to have as large an effective surface as possible renders preferable the use of rectilinear elements since these tend to have the best `packing factor` and hence provide the largest ON area in a display. The need to supply pixels, small in area relative to the size of the array, suggests, therefore the use of square pixels.
`LED` herein refers to the chip which is mounted in a lens, although the chip plus lens is elsewhere frequently called, collectively, an LED.
The invention therefore provides, in one aspect, a display element, or a column or array thereof where the stator is preferably an open front housing, usually of approximately square shape, and a disk designed to rotate about an axis to display a bright or dark side in the viewing direction. (The background to the disk, when viewed in the viewing direction is typically coloured to contrast with the bright disk side and to match the dark disk side). An LED associated with the stator is located and provided with shroud means so that the LED rays illuminate the bright side of the disk for viewers on each side of the viewing directions. Means prevents the escape of LED rays in directions transverse to the viewing direction.
The preferred drive for the disk is electromagnetic in accord with techniques well known to those skilled in the art. However, other drives may be used within the scope of the invention.
In a preferred form of the invention, a printed circuit board (`PCB`) is mounted on one side of the disk and an LED mounted so that its rays will illuminate the bright side of the disk when the latter is in the ON position. This will depend on the attitude of the disk when ON. Preferably in ON position, the bright side of the disk is tilted slightly toward the LED and the LED rays are directed transverse to the viewing direction. Means, preferably a housing on the sides of the LED lens (relative to the desired direction of LED rays); and the PCB on the back, prevent the escape of rays diverging very much from the desired LED ray direction. Sufficient divergence of LED rays from the desired LED ray disk-illuminating directionDexists that viewers on a side-walk in front of the bus (on one selected side) may view the direct rays from the LEDs) when the disk is in ON position.
The wall need not be a PCB, within the scope of the invention. However, it is considered most suitable since it is then convenient to provide metal strips on the PCB board, to provide the LED leads.
Preferably, only one side wall is provided rather than two for each element or column thereof. The opposed wall is then provided by the adjacent element or columns thereof in the direction of LED radiation.